1. Field of the Invention
This invention relates to well tools and more particularly to gas lift valves used in operating oil wells through practice of gas lift techniques.
2. Description of the Prior Art
Gas lift valves have been used for many years to control the injection of lift gas into a flow conductor from the exterior thereof to aerate a column of liquid therein and aid in lifting it to the surface. While fixed orifices have been used successfully in lifting liquids from well conduits, they waste lift gas. Gas lift valves, on the other hand, are more efficient in that they need not remain open and passing lift gas all the time, but may open only when conditions justify their efficient operation.
One type of valve used in gas lift operations is the differentially operated gas lift valve, commonly called "differential valve". Such valves are normally open but close upon occurrence of a condition where the difference in pressures between the gas lift column and the production column exceeds a predetermined value for which the gas lift valve has been adjusted to close. Because of this characteristic, the pressure of the lift gas is not critical once the well has been unloaded and placed on production. Generally, lift gas is injected into the tubing-casing annulus of a well through a choke which is sized to pass the quantity of lift gas which is needed to produce the desired volume of liquids from the well. In some cases, gas is injected into the tubing and well products are lifted through the annulus.
Listed here are several U.S. patents which disclose a variety of differential gas lift valves. They are: U.S. Pat. No. 2,144,144, U.S. Pat. No. 2,236,864, U.S. Pat. No. 2,256,704, U.S. Pat. No. 2,288,605, U.S. Pat. No. 2,305,250, U.S. Pat. No. 2,314,868, U.S. Pat. No. 2,323,893, U.S. Pat. No. 2,541,807, U.S. Pat. No. 2,588,715.
All of the above listed patents, with the exception of U.S. Pat. No. 2,305,250, have a common basic structural design which is, perhaps, more readily seen and understood when looking at U.S. Pat. No. 2,144,144 which issued to C. S. Crickmer on Jan. 17, 1939. In this patent, and in FIGS. 11 and 12 in particular, the valve member has a piston 41 having its upper end exposed to casing pressure transmitted into the valve housing through lateral ports 46, and has its lower end exposed to tubing pressure transmitted thereto through tubing port 25. A coil spring 44 biases the valve toward open position (shown in FIG. 11). When the pressure in the casing exceeds the pressure in the tubing by a sufficient margin, the difference between these pressures acting across the area of the piston 41 will create sufficient force to move the valve to closed position (shown in FIG. 12), as it overcomes the force of spring 44, and engages seat surface 47 with the seat to stop the flow of lift gas from the casing into the tubing. When the difference between the tubing and casing pressure is reduced sufficiently, the spring 44 will open the valve, i.e., return the valve to its open position (shown in FIG. 11). It is readily seen that the piston areas exposed to the tubing pressure and the casing pressure are equal. Since these areas are equal, the valve both opens and closes at substantially the same pressure. Understandably, the valve may quickly cycle between open and closed positions several times before assuming one position or the other. This is not desirable. It wastes gas and causes unnecessary wear and tear on the mechanism.
The gas lift valves disclosed in the other patents in this group also have valve means with pistons having equal areas exposed to tubing and casing pressures.
U.S. Pat. No. 2,256,704 which issued on Sept. 23, 1941 to C. S. Crickmer, et al., discloses a gas lift valve which is similar to those just discussed. In operation, this valve is in the position shown in FIG. 2 of the patent. Lift gas from the casing enters side ports 37, flows upwardly in flutes 36, flows around the upper portion of valve 30 and past the tapered upper end thereof, and from thence through port 27. When the pressure in the tubing decreases, the velocity of the gas passing the upper end of the valve increases, and the valve moves up toward the seat, pinching the flow therebetween. This further increases the velocity. The upper portion 34 of the valve acts as a piston and plunges into port 27 until it seats as shown in FIG. 3. When liquid rises sufficiently high in the tubing to create sufficient back pressure acting against the upper end of the valve (which acts like a piston), the valve will move to open position.
U.S. Pat. Nos. 2,288,605; 2,314,868; and 2,323,893 which issued to A. Boynton on July 7, 1942, Mar. 30, 1943, and July 13, 1943, respectively, each show a gas lift valve with a hollow-stemmed valve, but these valves have their opposite ends formed with equal seal surfaces thereon and are adpated to engage in equal cup-shaped seats at either end to stop flow through the hollow stem when the differential pressure across the mechanism creates a force exceeding the force of centering spring means associated with the tubular valve stem.
U.S. Pat. No. 2,305,250 which issued on Dec. 15, 1942 to H. U. Garrett et al., discloses a differential gas lift valve which indeed is operated by the difference between tubing and casing pressures, but its valve member does not always present equal areas to the tubing and casing areas and, therefore, acts differently from the devices of the patents just discussed.
The device of U.S. Pat. No. 2,305,250 has a valve member 21 (see FIGS. 2 and 3) with a conical seat surface 23, 22 on its upper and lower ends and a piston 28 just below the upper end. Spring 30 biases the valve toward open position (shown in FIG. 2). The upper end of the valve member can close upper seat 20 which opens to the casing, or its lower end can close lower seat 14 which opens to the tubing. Lateral ports 31 in the valve housing conduct lift gas from the casing into the housing and to lower seat 14 when the valve is open. The ports 31 have a combined area much smaller than the area of either port 14 or port 20.
When the valve is open and is seated on upper port 20, casing pressure cannot act upon the upper side of piston 28, but acts upon the upper end of the valve through port 20. Port 20 is smaller than port 14, and the piston is larger than port 14. Thus, when the difference between the casing and tubing pressures becomes sufficiently great to unseat the valve from upper seat 20, casing pressure immediately acts upon the greater area of piston 28, causing the valve to move to fully closed position with a "snap action". This imparts positive operation to the gas lift valve and prevents the unwanted cycling mentioned above. It also saves gas and avoids unnecessary wear and tear on the valve mechanism.
It is obvious that when the valve is open, the differential pressure acts upon the area of upper seat 20, and when it is closed, the differential pressure acts upon the larger area of lower seat 14. This difference in port sizes causes the gas lift valve to close at a first differential pressure value and to reopen at a second differential value which is lesser than the first.
The present invention is an improvement over the differential gas lift valves mentioned hereinabove. It is formed with unequal areas across which the differential pressure may act to actuate the valve to open or closed position so that the valve will close at a first differential pressure value and close at a lesser value without cycling and without need of a special piston to provide snap action because the opening and closing differentials are so different. Also, the present invention provides a gas lift valve having a less tortuous flow passage through it in that the flow passage passes straight through the valve. The valve stem is hollow and conducts lift gas to the ball valve closure member attached thereto, the lift gas exiting the stem through lateral ports at the ball and passing around the ball and through the seat. A choke or flow bean is advantageously provided in the hollow valve stem. Further, the present invention provides a gas lift valve having novel means for preventing backflow through it.
There is not found in the known prior art a gas lift valve having a valve stem hollow from end to end which closes at one differential pressure and reopens at a lesser differential pressure and having a straight flow passage therethrough with only a small but streamlined detour around its ball closure. Neither was there found a gas lift valve in the known prior art having a floating valve seat which also acts as a check valve and at the same time provdies a straight unobstructed flow passage therethrough when in open position. In addition, there was not found a gas lift valve having a hollow valve stem in which a flow restrictor is provided.
The present invention overcomes many of the problems associated with differential gas lift valves by providing gas lift valves having a straight-through flow passage, a hollow stem constituting a portion of that straight passage, a flow restrictor in the hollow stem, a floating valve seat which also serves to check against backflow, a closing differential which is higher than its opening differential, and simple construction which is less costly to manufacture.